The
digestive tract is organized around a long digestive tract that
begins at the mouth and ends at the anus. The tissue in this
tract can be divided into four layers:

Mucosa

The mucosa is an example of a mucous
membrane that consists of:

Mucosal epithelium

The type of epithelium
varies along the tract depending
upon function. It is stratified
to resist mechanical abrasion
and stress and simple in places
where there is secretion or
absorption. Plicae (sing.
plica), of folds, are
sometimes present to increase
surface area.

Lamina propria

This is the underlying
layer of areolar connective
tissue. It contains blood
vessels, nerve endings,
lymphatic vessels, and cells of
the immune system. It provides
mechanical and metabolic support
for the epithelium and
protection as part of the immune
system.

Muscularis mucosae

The outer boundary of the
mucosa is often marked by a band
of smooth muscle arranged in two
thin concentric layers, an inner
circular layer and outer
longitudinal layer. Contraction
of the muscularis mucosae
agitates the epithelium and
alters its shape.

Submucosa

The submucosa is the connective tissue
that surrounds the mucosa. Large blood vessels
and lymphatics and occasional exocrine glands
are found in this layer. A network of nerve
fibers and neurons cell bodies called the
submucosal plexus (Meissners plexus) is
present in this layer.

Muscularis Externa

This is a muscular layer that surrounds
the submucosa. It consists of an inner circular
layer and an outer longitudinal layer of smooth
muscle. The layers sandwich a myenteric
plexus (Auerbachs plexus) that contains
nerve fibers and parasympathetic ganglia. These
muscle layers mechanically process and propel
the contents of the digestive tract by
coordinated movements controlled by the neurons
of the myenteric plexus.

Regions along the tract where the
circular layer is thickened form sphincters that
control the flow of materials from one part of
the tract to the next.

Serosa

Where the digestive tract is free to move
within the abdominal cavity it is surrounded by
a serous membrane called the serosa.
It places where the tract is firmly attached
to surrounding structures the attaching
connective tissue is called adventitia.

Movement of Digestive Materials

Materials move
in the digestive tract by the following two types of movement:

Peristalsis

Peristalsis
is the coordinated contraction of the muscularis
that moves a bolus (mass of ingesta)
along the length of the digestive tract.

Segmentation

Segmentation
is coordinated contraction of the
muscularis that fragments and churns digested
materials with intestinal secretions there is
no net movement in either direction.

Peritoneum

The
serosa is also called the visceral peritoneum and is
continuous with the parietal peritoneum. The peritoneum
lines the peritoneal cavity within the abdominal cavity.
Abdominal organs have the following relationships with the
peritoneum:

Intraperitoneal organs

These
organs lie completely within the peritoneal
cavity and are covered on all sides by visceral
peritoneum, e.g., stomach, liver, ileum.

Retroperitoneal organs

These
organs are outside the peritoneal cavity and
only their anterior surface is covered by
visceral peritoneum, e.g., kidneys, ureters,
aorta.

Secondarily retroperitoneal organs

These
organs are similar to retroperitoneal organs in
their relationship to the peritoneal cavity but
acquire their position during development when
the visceral peritoneum on their posterior side
fuses with the parietal peritoneum.

Mesenteries

The
intraperitoneal organs are suspended in the peritoneal cavity by
fused double-sheets of peritoneal membrane called
mesenteries. Mesenteries provide a conduit for blood
vessels, nerves and lymphatics to the digestive tract and
stabilize the position of organs.

Mesenteries include:

Mesentery proper  The mesentery that
suspends the small intestines from the dorsal body wall.

Greater omentum  A mesentery containing a
thick layer of adipose tissue forms a pouch that extends
from the greater curvature of stomach to the transverse
colon.

Lesser omentum  A mesentery between the
lesser curvature of the stomach and the liver.

Oral Cavity

The digestive tract begins at the oral
cavity and performs 1) analysis, 2) mechanical processing, 3) lubrication, and
4) digestion.The oral cavity is lined by oral mucosa
which has stratified squamous epithelium that protects the
mucosa from abrasion.

The
mucosal surfaces within the oral cavity include:

Cheeks  form the lateral walls of the oral
cavity.

Labia (sing. labium)  the inner lining of
the lips.

Gingivae  the mucosa that covers the
alveolar processes and surrounds the neck of each tooth.

Hard and soft palate  form the roof of the
oral cavity.

Uvula  tissue that dangles from the
posterior margin of the soft palate.

Tongue  dominates the floor of the oral
cavity.

Spaces within the oral cavity include:

Vestibule  is the space between lips,
cheeks and teeth.

Fauces  is the passage between the oral
cavity and the pharynx.

Tongue

The tongue is divided into a body
(in the oral cavity) and a root (in the pharynx). The v-shaped line
formed by the circumvallate papillae indicate the boundary between the body and
the root. The superior surface of the tongue is called the dorsum.

The lingual frenulum is a thin
fold of mucous membrane that holds the body of the tongue to the oral floor.

The
tongue contains intrinsic muscles
that alter the shape of the tongue and extrinsic muscles that positions
the tongue in the oral cavity.

Salivary Glands

There are three pairs of salivary glands
that secret saliva into the oral cavity:

1. Parotid salivary glands  located
anterior to the outer ear, produces secretions that empties into the vestibule
near the second upper molar by the parotid duct.

2. Sublingual salivary glands  are
under the floor of the mouth and are drained by numerous sublingual ducts.

3. Submandibular salivary glands 
located on the medial side of the mandible under the mylohyoid line.
Submandibular ducts drain secretions through an opening on either side of
the lingual frenulum.

The saliva contains salivary amylase
which begins digestion of complex carbohydrates and mucins which are
glycoproteins that enhance the lubricating qualities of saliva. Saliva also
helps to control oral bacterial populations.

Teeth

Mastication, or chewing, is performed by
the teeth.

Tooth Anatomy

The bulk of the tooth is formed by a
bony substance called dentin. Cytoplasmic processes extend into the
dentin from cells in the pulp cavity. Highly vascular connective tissue
within the pulp cavity receives blood and sensation through blood vessels and
nerves that enter the root at the apical foramen and travel through the
root canal.

The tooth is anchored to the bony socket
of the alveolar process by collagen fibers of
the periodontal ligament. A bony
substance called cementum covers the dentin of the root and the fibers of
the periodontal ligament are anchored in cementum.

The crown is the visible portion of the
tooth above the gingivae. The dentin of the crown is covered by enamel,
the hardest material in the body. The neck is the boundary between the
crown and the root.

Types of teeth

There are four types of teeth:

Incisors  blade-like teeth for clipping
and cutting.

Cuspids (Canines)  conical with a
pointed tip for tearing and slashing.

Bicuspids (Premolars)  have one or two
roots and flattened crowns with prominent ridges for crushing, mashing and
grinding.

Molars  have three or more roots and
broader crowns with ridges for crushing and grinding.

Dental Succession

Twenty primary or deciduous
teeth consist of two incisors, one cuspid and two molars on each side. The
primary teeth are replaced with the secondary or permanent dentition
consisting of two incisors, one cuspid, two bicuspids, and three molars on
each side.

Dental Frame of Reference

The various surfaces of the teeth are
designated as follows:

Labial or buccal surface is the
surface that faces the lips or cheeks.

Palatal or lingual surface is the
inner surface facing the tongue.

Mesial surface is the medial or anterior
surface.

Distal surface is the lateral or
posterior surface.

Occlusal surface is the surface that
comes into contact with the opposing tooth during chewing.

Pharynx

The pharynx serves as a common
passageway for food, liquids and air.

Esophagus

The
esophagus is a muscular tube approximately 1 foot long that
transports food and liquids from the pharynx to the stomach.Some of the distinctive histological features of the
esophagus are:

The epithelium of the mucosa is stratified squamous
to resist abrasion.

The submucosa has esophageal glands that produce
mucus for lubrication.

The muscularis externa consists of skeletal muscle
in its upper third, mixed skeletal and smooth muscle in
its middle third, and smooth muscle in its lower third.

The outer layer of the esophagus is
adventitia that anchors the esophagus against the dorsal body wall.

Stomach

The functions of the stomach include:

1. bulk storage of food

2. mechanical breakdown of food

3. chemical digestion of food

Anatomy of the Stomach

The stomach is an expanded J-shaped
segment of the digestive tract. The medial surface of the stomach has a
lesser curvature and the lateral surface has a greater curvature and
the anterior and posterior surfaces are smooth and rounded.

The stomach can be divided into 4 regions:

1. Cardia  is the region of the stomach
immediately surrounding the junction of the esophagus with the stomach.

2. Fundus  is the region of the stomach
superior to the gastroesophageal junction.

3.Body  is the largest region of the
stomach and lies between the fundus and the lower curving segment of the J.

4.Pylorus  is the curve of the J. A
muscular pyloric sphincter regulates the release of chyme into the
duodenum.

In the contracted stomach the mucosa is thrown into
folds called rugae. As the stomach expands the rugae flatten out to
allow the stomachs expansion.

Mesenteries of Stomach

The visceral peritoneum of the anterior and posterior
surfaces fuse at the lesser and greater curvatures to
form two prominent mesenteries:

Greater omentum  forms a pouch that
hangs like an apron between the anterior abdominal wall and the abdominal
viscera.

Lesser omentum  is the mesentery
between the lesser curvature and the liver.

Musculature of the Stomach

Both the muscularis mucosae and
muscularis externa have extra muscle layers. The muscularis mucosae has an extra
outer circular layer and the muscularis externa has an extra inner oblique
layer.

Histology of the Stomach

The mucosal epithelium of the stomach is
simple columnar and consists of mucus-secreting cells that produce a carpet of
mucus. Gastric pits open on the surface of the mucosa and are connected
to tubular gastric glands at their base. The neck is the place where the
gastric glands and gastric pits join and the mucosal neck cells at this
location continually divide to replace the cells that are lost.

Gastric Secretory Cells

The gastric glands produce
gastric juice and have three secretory cells:

The small intestines is where digestion
and absorption of nutrients primarily occurs. The surface area of the intestinal
mucosa is increased by transverse folds called plicae circulares. The
intestines is about 6 m (about 20 ft in length) and is divided into three
regions:

Duodenum

This initial segment is the shortest
(about 25 cm or 10 inches long) and the widest segment. Except for a small
segment where it is attached to the pylorus of the stomach, it is secondarily
retroperitoneal. It receives chyme from the stomach and digestive secretions
from the pancreas and the liver.

The
submucosa of the duodenum contains
duodenal (Brunners) glands that produce copious mucus containing
bicarbonate ion that rapidly changes the pH of the chyme coming from the stomach
and protects the lining of the duodenum from its
initial acidity.

Jejunum

The jejunum begins where the duodenum
abruptly bends and becomes intraperitoneal. The jejunum is about 2.5 m (8 ft)
long. The bulk of chemical digestion and nutrient absorption occurs here.
Isolated lymphoid nodules are present in the lamina propria.

Ileum

The last segment of the small intestines
is about 3.5 m (12 ft) long and ends where it joins the cecum at the sphincter
called the ileocecal valve. Both the jejunum and ileum are supported by a
mesentery called the mesentery proper. Lymphoid nodules are more numerous
in the lamina propria of the ileum and may form aggregated lymphoid nodules
(Peyers patches).

Histology of the Small Intestines

The mucosa of the small intestines has
finger-like projections called villi (sing. villus). The epithelial cells
of the villi have microvilli on their apical surface. Goblet cells
that secrete mucins are found among the epithelial cells. The plicae
circulares, villi and microvilli increase the surface area
enormously.

Tubular intestinal glands are found
between the villi extending into the lamina propria. Cells in the
intestinal glands continually divide and migrate onto the villi where they are
eventually shed at the tips of the villi.

The lamina propria of the small intestines have
lymphatic vessels called lacteals that transport absorbed lipids to the
circulation.

Large Intestines

The large intestines is about 1.5 m (5
ft) long and frames the small intestines. Its major functions include:

1. Reabsorption of water, electrolytes and
compaction of feces.

2. Absorption of vitamins.

3. Storage of fecal material.

The large
intestines can be divided into three parts:

Cecum

The cecum is an expanded pouch that
receives ingesta from the ileum through the ileocecal valve in its medial wall.
The appendix is attached to the cecum and contains numerous lymphoid nodules.

Colon

The colon makes up most of the large
intestines and has several distinctive features:

The ascending and descending colons are
secondarily retroperitoneal on the right and left sides of the peritoneal
cavity, respectively. The transverse and sigmoid colons are intraperitoneal and
are suspended by mysentery.

Rectum

The rectum is the last 15 cm (6 in) of
the large intestines. It temporarily stores feces prior to excretion. The last
portion of the rectum is the anal canal which ends at the anus
where the epithelium changes from simple columnar to stratified squamous.
Distention of veins in the lamina propria and submucosa of the anal canal are
called hemorrhoids.

Histology of Colon

Distinctive histological features of the
colon include:

1. Absence of villi.

2. Goblet cells are abundant

3. Intestinal glands are deep and contain
numerous goblet cells

4. Presence of large, numerous lymphoid
nodules

5. Longitudinal layer of muscularis externa have three
thickened bands corresponding to the taeniae coli.

Accesory Digestive Organs

Liver

The liver is the largest visceral organ
and has more than 200 different functions that fall in one of three categories:

3.Synthesis and secretion of bile 
bile helps neutralize acidic chyme from the stomach and enables digestion of
lipids in the small intestines.

Anatomy of the liver

On the anterior surface of the liver the
falciform ligament, a ventral mesentery, separates the right
and left lobes of the liver. The round ligament is found in
the inferior margin of the falciform ligament. The liver is attached to the
diaphragm by the coronary ligament.

The lobes of the liver can be divided
into segments based on the blood supply and secretory ducts. The segments have
clinical significance, particularly for surgery.

Blood supply to the liver

The liver has two sources of blood:
hepatic artery proper which delivers oxygenated blood and hepatic portal
vein which delivers blood containing nutrients from the intestines. The
hepatic veins drain blood from the liver and delivers it to the inferior
vena cava.

Histological organization of the liver

The parenchyma of the liver is divided
by connective tissue into liver lobules. The hepatocytes are
arranged in plates one cell thick that are arranged like the spokes of the wheel
around the central vein. The spaces between the plates contain
sinusoids that have in addition to endothelial cells, phagocytic Kupffer
cells.

Blood enters the lobule through the
portal areas at the corners of the lobules that contain:

1. Branches of the hepatic portal vein

2. Branches of the hepatic artery
proper

3. Branches of the bile duct

Blood that is drained by the central
vein drains into hepatic veins that drain into the inferior vena cava.

Bile secretion

Bile is secreted into channels formed by
opposing hepatocyte cell membranes called bile canaliculi. The bile is
collected by bile ductules that connect to the bile ducts of the
portal areas.

The right and left ducts collect
the bile secreted by their respective liver lobes. These ducts combine to form
the common hepatic duct. The common hepatic duct fuses with the cystic
duct to form the common bile duct.

Gall Bladder

The gall bladder is a hollow
pear-shaped, muscular organ that stores and concentrates bile. Between meals,
bile secreted by the liver enters the gall bladder through the cystic duct.
Under the stimulation that occurs during a meal, bile is ejected from the gall
bladder into the cystic duct which fuses with the common hepatic duct to form
the common bile duct which opens into the duodenum at the duodenal papilla.

Pancreas

The pancreas is secondarily
retroperitoneal and is bound to the posterior wall of the abdominal cavity. It
can be divided into a broad head that lies within the loop of the
duodenum, a body, and a blunt rounded tail that lies near the
spleen.

The
pancreas is primarily an exocrine
organ producing digestive enzymes and buffers and is secondarily an endocrine
organ. The pancreatic exocrine secretions are delivered to the duodenum by a
large pancreatic duct which joints the common bile duct at the
duodenal ampulla. A small accessory pancreatic duct may branch from
it and empty its secretion separately at the lesser duodenal papilla.